1. Field of the Invention
The present invention relates to a flexible meshing type gear drive, and in particular to a flexible meshing type gear drive having a flexible external gear capable of realizing a low reduction ratio.
2. Related Art Statement
A typical flexible meshing type gear drive comprises a circular rigid internal gear, a flexible external gear arranged inside the circular rigid internal gear, and a wave generator for deforming the flexible external gear into, for example, an ellipsoid to mesh it with the rigid internal gear partially. The wave generator is driven by a motor to move the meshing portions between the gears circumferentially, whereby relative rotation therebetween is occurred in accordance with the difference in tooth number between the gears.
The circular rigid internal gear is normally fixed to a device housing (not shown) wherein relative rotation is output from the flexible external gear. Typically, the difference in tooth number between the gears is 2n (n: positive integer), and, for example, the flexible external gear has the external teeth, the number of which is two fewer than that of the circular rigid internal gear. In this case, the reduction ratio of the flexible meshing type gear device 1 is expressed by the following equation.
______________________________________ i = 1/R = (Zf-Zc) / Zf = -2 / Zf ______________________________________
where
Zc: tooth number of the circular rigid internal gear, and PA1 Zf: tooth number of the flexible external gear.
Provided that Zc=202 and Zf=200, the reduction ration i becomes -1/100 in which symbol `-` means that the rotational direction of the output is opposite against that of the rotational input, while symbol `+` means that the output rotational direction is the same as the input rotational one.
As the reduction ratio of a flexible meshing type gear device becomes low, the amount of deformation of the flexible external gear along the radial direction becomes large, whereby excessive stress concentration occurs in the bottom portion of the external teeth of the external gear. Thus, a flexible meshing type gear device having a low reduction ratio must be provided with a flexible external gear of the type having a high fatigue strength. If it cannot assure the sufficient fatigue strength of a flexible external gear, a flexible meshing type gear drive having a low reduction ratio cannot be realized practically.